Antennas for communication devices, especially portable communications receivers, such as pagers, have generally been restricted to using electromagnetic loop antennas which optimize signal reception when the receivers are worn on the body. While loop antennas have performed satisfactorily for many years, the newer generations of personal portable communication devices are becoming ever smaller and their use is no longer limited to use on the body.
The size of communication devices has imposed strict space demands on the antennas capable of being utilized in such devices. To compensate for the decrease in available space, one known antenna interposes a dielectric core within a center fed loop antenna. The resulting antenna is responsive to both the magnetic and electric fields of an electromagnetic wave, thus improving the sensitivity of the antenna for a given size. It is also known to integrate a slot antenna into the communication device so that it forms a part of the housing where the slot antenna includes three plates arranged generally to have a U-shaped cross-section. A patch antenna has also been used in portable communications devices, a patch antenna being advantageous because of its generally low profile. Such patch antennas typically include (a) a thin flat metallic region typically called the patch; (b) a dielectric substrate; (c) a ground plane, which is usually much larger than the patch; and (d) a feed which supplies or receives the RF power.
The physical size of a patch antenna is determined by numerous factors. The principal factor that determines the antenna size is the frequency at which the antenna is to resonate. At higher operating frequencies, the patch antenna is small in size. The antenna's size, however, must be increased as the operating frequency is lowered. The patch antenna size is also influenced by other factors as well. For example, the electrical length of the patch is directly related to the dielectric constant of the dielectric substrate. A high dielectric constant substrate results in an antenna having an effective electrical length that is longer than the same antenna would have if the dielectric constant were lower. Another factor affecting the effective electrical length of the antenna is the ground plane. The smaller the effective size of the ground plane, the longer the patch element must be to operate properly at a given operating frequency. The effect of the ground plane on patch size is particularly noticeable in small portable communication devices where the space available for an adequate ground plane structure is limited. Although a dielectric substrate having a higher dielectric constant may be chosen to compensate for the less than optimal ground plane characteristics, such an approach tends to raise the Q factor of the antenna and, thus, decrease the operating bandwidth of the antenna. A narrow bandwidth may not be appropriate for many types of communications devices.